138 BACTERIOLOGY 



Preservation of Blood Serum. It is sometimes desirable 

 to preserve blood serum in a fluid state. This can be done 

 by the fractional method of sterilization at low tempera- 

 tures, already described, or with much less effort, and with- 

 out the use of heat, by a method that we have found very 

 satisfactory. In the course of Kirschner's investigations 

 chloroform was shown to possess decided disinfectant 

 properties; as it is quite volatile, it is easily got rid of when 

 its disinfectant or antiseptic properties are no longer required. 

 If, therefore, the serum to be preserved be placed in a closely 

 stoppered flask and enough chloroform added to form a 

 thin layer, about 2 mm., on the bottom, the serum may 

 be kept indefinitely without contamination, so long as the 

 chloroform is not permitted to evaporate. This latter pro- 

 vision is one on which success depends. If the vessel con- 

 taining the mixture of chloroform and serum be not tightly 

 corked, the chloroform vapor escapes pretty rapidly and 

 exerts no preservative action. In fact, bacteria will grow 

 uninterruptedly in a cotton-stoppered test-tube containing 

 bouillon to which chloroform has been added. When re- 

 quired for use, the serum is decanted into test-tubes, which 

 are then placed in a water-bath at about 50 C. until all 

 the chloroform has been driven off; this can be determined 

 by the absence of its characteristic odor. The serum may 

 then be solidified, sterilized by heat, and employed for 

 culture purposes. We have found serum so preserved to 

 answer all requirements as a culture medium. 



Milk. Fresh milk should be allowed to stand overnight in 

 an ice-chest, the cream then removed, and the remainder of 

 the milk pipetted into test-tubes, about 8 c.c. to each tube, 

 and sterilized by the intermittent process, at the tem- 

 perature of steam, for three successive days. 



